1. Field of the Invention
This invention relates to the field of regasification systems, and more specifically to a method and system for regasification of liquid natural gas (LNG), intended for installation on a seagoing vessels.
2. Description of the Related Art
Natural gas exists in subterranean reservoirs throughout the world. This gas (i.e. methane) is a valuable commodity, and various methods and equipment exist for the extraction, treatment and transportation of this natural gas from its reservoir to the consumer. The simplest transportation means is a pipeline for conveying the gas in its gaseous state from the reservoir to the consumer. In many instances, however, the reservoirs are located in remote areas and/or areas with restricted accessibility, such that laying a pipeline is either technically very complicated and/or economically unprofitable. One very common technique for transporting natural gas from such areas, is to liquefy the natural gas at or near the extraction site, and transport this liquefied natural gas (LNG) to the market in specially designed storage tanks, often placed aboard a sea-going vessel.
The process of liquefying the natural gas involves compression and cooling of the gas to cryogenic temperatures (e.g. −160° C.). The LNG carrier may thus transport a significant amount of liquefied gas to its destination. At this destination, the LNG is offloaded to special tanks onshore, before it is either transported by road or rail on LNG carrying vehicles or revaporized and transported by e.g. pipelines.
It is, however, in many instances more advantageous to revaporize the natural gas aboard the seagoing carrier before the gas is off-loaded into onshore pipelines. U.S. Pat. No. 6,089,022 (Zednik et al.) discloses such a system and method for regasifying LNG aboard a carrier vessel before the re-vaporized natural gas is transferred to shore. The LNG is flowed through one or more vaporizers positioned aboard the vessel. Seawater taken from the body of water surrounding the carrier vessel is flowed through a vaporizer to heat and vaporize the LNG back into natural gas before this natural gas is offloaded to onshore facilities.
Zednik et al. furthermore cites the “TRI-EX” Intermediate Fluid-type LNG vaporizer as one vaporizer type capable of using seawater as the principal heat exchange medium. Such type of vaporizer is disclosed in U.S. Pat. No. 6,367,429 (assigned to Kabushiki Kaisha Kobe Seiko Sho) and comprises in principle a housing with a pre-heat section and a final heating section. The pre-heat section has a plurality of pipes running through it which fluidly connect two manifolds which lie at either end of the pre-heat section, while the final heating section has also a plurality of pipes running through it which fluidly connect two other manifolds at either end of the final heating section. Seawater, which is collected directly from the sea surrounding the vessel, is pumped into a manifold and flows through the pipes in the final heating section and into the manifold before flowing through the pipes in the pre-heat section and into the manifold, from which the seawater is discharged back into the sea.
In operation, the LNG flows from a booster pump and into a looped circuit which is positioned within the pre-heat section of the vaporizer, which in turn contains a “permanent” bath of an evaporative coolant (e.g. propane) in the lower portion. The seawater, flowing through the pipes, will “heat” the propane in the bath, causing the propane to evaporate and rise within the precooling section. As the propane gas contacts the looped circuit, it gives up heat to the extremely cold LNG flowing through the circuit and recondenses to fall back into the bath, thereby providing a continuous, circulating “heating” cycle of the propane within the pre-heat section.
Problem to be Solved by the Invention
While the present regasification systems, like the one cited above, work well under given conditions, their use and applicability are nonetheless restricted by certain limitations and disadvantages. It is for example not possible to control the condensation pressure in the known systems. Furthermore, the evaporative coolant (e.g. propane) is in the known systems allowed to evaporate and condense in an unrestrained fashion; the heat transfer process is thus relatively slow and—in order to achieve optimum system efficiencies—large volumes are required. This often leads to the regasification systems being very large and requiring a great portion of valuable deck space.
It is therefore a long felt need for a regasification system which allows the condensation pressure to be more easily controlled, and a system which is more compact and flexible in operation than known regasification systems.
Means for Solving the Problem
The present invention solves that need by providing a novel regasification method and system in which, when in operation, the evaporative coolant is forced through an evaporation and condensation cycle; and allowing control of the evaporative coolant condensation pressure; thus yielding a more flexible and more compact regasification system than those of the prior art.